Allow sign up to be disabled via configuration

There are many instances where you don't want users to be able to sign
up (e.g. admin-only system). Clearance can now disable sign up by
setting:

    Clearance.configure do |config|
      # other configuration options
      config.sign_up = false
    end

in `config/initializers/clearance.rb`.
Fix style issues
More style fixes
Change config.sign_up to config.allow_sign_up
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